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Perfluoropolyether oils with exceptionally high thermal stability are useful for extreme conditions. Greases are made by the addition of thickening agents, for instance, finely divided silica, boron nitride, clay, soaps, or poly(tetrafluoroethylene) to oils. Both oils and greases tend to corrode metals at elevated temperatures, thus anticorrosion additives are included. An example of such an anticorrosion additive is disclosed in U.S. Pat. No. 6,184,187. The stability of this phosphorus-containing anticorrosion additive has an upper temperature limit in service of about 180° C., whereas the perfluoropolyethers are stable at temperatures over 300° C. Though U.S. Pat. No. 6,653,511 discloses perfluoropolyether primary bromides and iodides useful, for example, to prepare lubricants, surfactants, and additives for lubricants and surfactants, development of more thermally stable anticorrosion additives that extend the use at higher temperatures without corrosion occurring is highly desirable.